Make a hollow Minecraft build solid
For 3D Printing, what you need is a &quot;watertight&quot; or &quot;manifold&quot; model. Both terms mean that there are no holes in the exterior of the geometry, but more importantly, no internal faces, which is what you have here.
A great addon for checking your model is the 3D Print Toolbox, which comes standard in Blender, you just need to turn it on!
The simplest way to fix this is to use a Remesh modifier, this will recalculate the object into &quot;voxels&quot; (3D cubes) of a specific size that you can define based on the exterior. This, however, is VERY CPU heavy! It's REALLY slow to calculate!
There is a MUCH better remesh option though. It lives either in the tools panel in the sculpting mode, or under the Remesh tab in the Object Data Properties tab.

0.1mm setting is more than good enough for 3D Printing. But be warned, this is still quite CPU intensive!
For your model, you may need to leave it for a few hours depending on how powerful your PC is!